1999 Volvo S70 Ignition

Other Category problem
1999 Volvo S70 Engine Size unknown Front Wheel Drive Automatic 115000 miles

Sometimes, when I insert the key to start the car, the key just spins around. I can go 360 degrees, sometimes several times. So far, I have been able to get the key to "catch" and am able to start the car.

What is the problem and what is the solution? How expensive a situation is this? Thank you!